CGI on Ang Lee's Hulk was very good, but the design of the Hulk just wasn't right. It was a good attempt though, the first CGI'ed version of Hulk.
The design in The Incredible Hulk was better, closer to the comics, but the CGI wasn't very good.The Avengers version was just the right mixture of both, great design and great CGI.
My opinion, of course.

Avengers Hulk wasn't a mix. They scraped all previous designs of the Hulk and decide to start from scratch. They then selected one of the comic book versions of Hulk and made there Avengers Hulk look as close to that version as possible. After that was done they incorporated some of Mark Ruffalo's facial features into the face of there Avengers Hulk with out compromising Hulk's look.

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Quote:

What is the most indestructable thing in the avengers? Ironman's suit, Captain America's Shield, or Thor's Hammer?﻿ The correct answer is Hulk's Pants